SiH Species Research Articles

In situ Spectroelectrochemical Study of the Anodic Dissolution of Silicon by Potential-Difference and Electromodulated FT-IR Spectroscopy

The anodic dissolution of p-Si has been investigated by in situ infrared spectroscopy. The combination of potential-difference and electromodulated spectroscopies allows for the acquisition of a rather complete picture of the various regimes of the dissolution. After a review of general principles for studying electrochemical interfaces, a study of the interfacial oxide layer formed in the electropolishing regime is presented. Quantitative analysis shows that the thickness and quality of the oxide (density and defect content) depend upon electrode potential. Free-carrier absorption detected in electromodulated spectra shows that the blocking character of the oxide is correlated with the buildup of a stoichiometric oxide of low defectivity at sufficiently positive potentials. Furthermore, the dynamic response to the modulation reveals that oxides formed at weak positive potentials interact with electrolyte species through electro-induced adsorptions/desorptions on charged SiOH sites. At more positive potentials, charge is transported across the oxide by charged defects which could be associated with tricoordinated, positively charged SiO species. Finally, results obtained during porous silicon formation at weak positive potentials are presented. Potential-difference spectroscopy indicates that the electrode exhibits a very large specific surface area, and that the surface is covered by SiH bonds. Electromodulated infrared spectroscopy reveals that the SiH species are generated upon anodic current flowing and that the breaking of these bonds is the rate-limiting step of the anodic reaction. These unexpected results have given rise to the elaboration of new microscopic models for the direct anodic dissolution of silicon in fluoride electrolytes.

Coverage-dependent thermal reactions of digermane on Si(100)-(2 x 1).

In this study, we examine the adsorption and thermal reactions of digermane (${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$) on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface with high-resolution core-level photoemission spectroscopy using synchrotron radiation. At 325 K, the digermane dissociatively chemisorbed to produce ${\mathrm{GeH}}_{3}$, ${\mathrm{GeH}}_{2}$, GeH, and SiH species. The sticking coefficient at zero coverage deduced from the photoemission intensity is \ensuremath{\sim}0.5. Successive annealing of the digermane-saturated surface to higher temperatures causes further decomposition of ${\mathrm{GeH}}_{3}$ and ${\mathrm{GeH}}_{2}$ and the desorption H from GeH and SiH, leaving atomic Ge on the surface. In light of the sufficiently large chemical and surface shifts in their core-level binding energies for different surface species, those processes were identified by examining the evolution of Ge 3d and Si 2p line shapes. Experimental results indicate that the reaction for H release from GeH not only occurred in a large temperature range but also depended heavily on the ${\mathrm{Ge}}_{2}$${\mathrm{H}}_{6}$ adsorption coverages. Two reaction routes for H release from Ge sites were used to describe the large reaction temperature spreads accurately. GeH decomposition by transferring the H atom to a surface Si dangling bond took place for low coverages at \ensuremath{\le}590 K, and ${\mathrm{H}}_{2}$ thermal desorption occurred for higher coverages in the range of 590--770 K. The former process of atom transfer of H from Ge to Si sites was directly observed in the Ge 3d and Si 2p photoemission spectra. \textcopyright{} 1996 The American Physical Society.

Surface roughening and metastable superstructures on wet-processed Si(111) surface induced by hydrogen desorption

Scanning tunneling microscopy (STM) has revealed surface morphology changes of the hydrogen (H)-terminated Si(111) surface accompanied by hydrogen desorption. The atomically flat and H-terminated surface with regular straight steps was prepared by NH 4F anisotropic etching. These straight step edges became rough when the surface was annealed at 490°C and then the H atoms were desorbed from the monohydride. This phenomenon was compared with previous reports on the behavior of SiH x clusters on the Si(111)-1 × 1:H formed on the Si(111)-7 × 7 surface. It was found that the model of the preferential detachment of SiH species from the step edges proposed for the latter case can also explain the phenomenon observed in the current experiment. The SiH species migrated over T 4 sites on the surface and formed various superstructures, such as 2 × 2, c(2 × 4), and √3 × √3, after the desorption of H atoms. These superstructures might be considered to be metastable ones, and were converted into 7 × 7 structures with almost no defects after annealing at 700°C.

HMDSO plasma polymerization and thin film optical properties

Thin films were deposited from hexamethyldisiloxane (HMDSO) in a glow discharge supplied with radiofrequency (rf) power. Actino-metric optical emission spectroscopy was used to follow trends in the plasma concentrations of the species SiH (414.2 nm), CH (431.4 nm), CO (520.0 nm), and H (656.3 nm) as a function of the applied rf power (range 5 to 35 W). Transmission infrared spectroscopy (IRS) was employed to characterize the molecular structure of the polymer, showing the presence of Si-H, Si-O-Si, Si-O-C and C-H groups. The deposition rate, determined by optical interferometry, ranged from 60 to 130 nm/min. Optical properties were determined from transmission ultra violet-visible spectroscopy (UVS) data. The absorption coefficient α, the refractive index n, and the optical gap E 04 of the polymer films were calculated as a function of the applied power. The refractive index at a photon energy of 1 eV varied from 1.45 to 1.55, depending on the rf power used for the deposition. The absorption coefficient showed an absorption edge similar to other non-crystalline materials, amorphous hydrogenated carbon, and semiconductors. For our samples, we define as an optical gap, the photon energy E 04 corresponding to the energy at an absorption of 10 4 cm −1. The values of E 04 decreased from 5.3 to 4.6 as the rf power was increased from 5 to 35 W.

Atomic-hydrogen-induced restructuring of the Si(100)(2 × 1)-K surface

The interaction of hydrogen atoms with the Si(100)(2 × 1)-K surface has been studied by using thermal desorption and electron energy loss spectroscopy. We have found that hydrogen adsorption induces the formation of potassium hydride islands at 90 K for even one-monolayer coverage of K, indicative of restructuring of the K-covered surface induced by hydrogen adsorption. The proposed reaction mechanism is as follows: incoming H atoms react with surface SiK species to generate surface SiH species and K atoms with the K atoms subsequently reacting with incoming H atoms to form potassium hydride. We discuss the relevance of the present results to the reaction of the metal-modified Si surface with hydrogen atoms.

In Situ Detection of Surface SiHn in Synchrotron-Radiation-Induced Chemical Vapor Deposition of a-Si on an SiO2 Substrate

The sensitivity and linearity of infrared reflection absorption spectroscopy (IRAS) has been significantly improved by using a buried-metal-layer (BML) substrate having an SiO(2)(15 nm)/Al(200 nm)/Si(100) structure, instead of a plain Si(100) substrate. By applying this BML-IRAS technique to the in situ observation of synchrotron-radiation-induced chemical vapor deposition of amorphous Si (a-Si) on an SiO(2) surface using Si(2)H(6) gas, the vibrational spectra of surface SiH(n) species in this reaction system have been observed for the first time with sufficient sensitivity for submonolayer coverage. The main silicon hydride species after deposition at 423 K are surface SiH(2) and SiH. Surface SiH(3) and SiH(2) are observed to be easily decomposed by synchrotron radiation irradiation. The decomposition rate of SiH by synchrotron radiation irradiation is much slower than those of SiH(2) and SiH(3).

A systematic study of the initial electrical and radiation hardness properties of reoxidized nitrided oxides by rapid thermal processing

The initial electrical and radiation hardness properties of MOS capacitors with reoxidized nitrided oxides (RNO) structures are systematically investigated by changing the pressure, temperature, and times of nitridation and reoxidation in rapid thermal processes. It was found that the initial flat-band voltage (V/sub fb/) and midgap interface trap density (D/sub itm/) are strongly dependent on the growth conditions and show concave or convex "turnaround" dependency on some process parameters. This may be explained by the hydrogen evaporation and oxygen passivation mechanisms. The radiation induced flat-band voltage shift (/spl Delta/V/sub fb/) and midgap interface trap density shift (/spl Delta/D/sub itm/) are also growth-condition dependent and show different "turnaround" dependencies on some process parameters from those observed in initial properties. This may be explained by the variations of the amount of hydrogen-related species such as Si-NH, Si-H, or Si-OH, and nitrogen-related species, such as Si-N, in the oxide bulk and at the Si/SiO/sub 2/ interface. Finally, the sample with a reoxidation pressure of 250 torr, a reoxidation temperature of 1050/spl deg/C, and a reoxidation time of 100 s is suggested to be the most radiation-hard together with good initial properties for RNO devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Etching of Si(111)-(7×7) and Si(100)-(2×1) surfaces by atomic hydrogen

We present scanning tunneling microscopy (STM) studies of the etching behavior of the surfaces of Si(111)-(7×7) and Si(100)-(2×1) by atomic hydrogen. Etching proceeds via the formation of volatile SiH4 through a two-step mechanism in which the H atoms react with SiH2 and SiH3 species on the surface at room temperature. By measuring the area of etch pits in the STM images taken under the same flux conditions, the etching of Si(100) is determined to be nearly three times faster than that of Si(111) because intermediate hydride products are more readily stabilized on the (100) surface.

ENHANCED REACTIVITY OF PENTACOORDINATED SILICON SPECIES. AN AB INITIO APPROACH

Abstract Molecular orbital calculations using GAUSSIAN 86 on tetracoordinated silicon species SiH n F4-n are compared with calculations on the anionic species formed by the addition of fluoride ion [SiH n F5-n ]. Similar calculations on isoelectronic tetra- and pentacoordinated phosphorus species [PHnF4-n ]+ and PH n F5-n are also carried out. All bond lengths in the pentacoordinated species are longer than in the related tetracoordinated species, with the greater increase in bond lengths occurring in the axial positions from both the silicon and phosphorus series. Fluoride ion addition results in only slight changes in the positive charge on silicon and phosphorus and in an increase in negative charge on all substituents. These calculations are used to elucidate the enhanced reactivity of pentacoordinated silicon observed in nucleophilic reactions.

Hydrogen desorption from Si(100)(2 × 1)-H induced by potassium adsorption

The interaction of K with Si(100)(2 × 1)-H has been studied by using thermal desorption and electron energy loss spectroscopy. We have found that above 360 K potassium adsorption induces concomitant H 2 desorption. This mechanism has been ascribed to a two step process consisting of the reaction of an incoming K atom with surface SiH species to form a surface complex, followed by the decomposition of this complex to give rise to the H 2 desorption. We have found that the isolated reaction intermediate, whose decomposition leads to the H 2 desorption around 450 K, is formed by the deposition of a small amount of K at 90 K. For the deposition of a large amount of K, the reaction intermediates form two types of potassium hydrides upon annealing and give rise to sharp H 2 desorption peaks at 360 and 380 K. Analysis of the K uptake curves above 360 K in terms of the Kisliuk model shows that K adsorption occurs via a precursor-mediated process, which is similar to that on clean Si(100)(2 × 1) but with a much low probability ( < 1 10 ) for the precursor to chemisorb. We suggest that the low probability for chemisorption is due to the activation barrier, estimated to be ∼ 3 kcal/mol, for the precursor K atoms to react with surface SiH species.

Kinetics of hydrogen desorption on a Si(111) surface

An in situ observation of hydrogen (H) desorption on a Si(111) surface was carried out by using high temperature scanning tunneling microscopy (STM). There appeared a disordered phase with no atomic images around 490°C at which the surface monohydride started to desorb. When the sample temperature was maintained at a certain moment above 490°C, the area of the disordered phase decreased with time and changed into a Si(111)-√3 × √3 R30° structure. This behavior of the surface structure can be explained by the migration of SiH species on an unreconstructed Si(111)-1 × 1 surface at elevated temperature. Based on this behavior, we propose a model for the H desorption.

Adsorption and decomposition of trichlorosilane and trichlorogermane on porous silicon and Si(100)2×1 surfaces

The adsorption and decomposition of trichlorosilane (SiHCl3) and trichlorogermane (GeHCl3) on silicon surfaces were studied using Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption investigations. The FTIR transmission spectroscopy experiments were performed in situ in an ultrahigh vacuum chamber on high surface area porous silicon samples. The FTIR spectra revealed that SiHCl3 dissociatively adsorbed at 200 K and formed SiH, SiClx, ClSiH, and Cl2SiH surface species. The presence of ClxSiH species indicated incomplete decomposition of SiHCl3 upon adsorption at 200 K. GeHCl3 also dissociatively adsorbed at 200 K and formed SiH and SiClx species. The absence of an infrared absorption in the Ge–H stretching region suggests complete transfer of hydrogen from Ge to surface Si atoms at 200 K. The thermal stabilities of the surface species were then studied with annealing experiments. The ClxSiH species deposited by SiHCl3 exposure at 200 K decomposed between 200 and 590 K and formed additional SiH and SiCl species. The SiClx (x=2 or 3) species deposited by either GeHCl3 or SiHCl3 dissociative adsorption converted to silicon monochloride species between 200 and 600 K. The SiH surface species subsequently decreased at temperatures between 680 and 780 K as H2 desorbed from the silicon surface. FTIR spectroscopy was also utilized to monitor the adsorption of SiHCl3 and GeHCl3 on porous silicon at different surface temperatures. These adsorption studies were in agreement with the thermal annealing experiments. Temperature programmed desorption (TPD) studies were performed following saturation SiHCl3 and GeHCl3 exposures on Si(100)2×1. After SiHCl3 adsorption, the only desorption species were H2, HCl, and SiCl2, and they were observed at 810, 850, and 970 K, respectively. Following GeHCl3 adsorption, the TPD experiments monitored H2, HCl, SiCl2, and Ge at 810, 850, 950, and 1200 K, respectively. The adsorption kinetics of SiHCl3 and GeHCl3 were also measured on Si(100)2×1 and initial reactive sticking coefficients of S0≊.019 for SiHCl3 and S0≊1 for GeHCl3 were determined at 500 K. These experiments provide insight into the surface chemistry of chlorosilanes and chlorogermanes during Si, Ge, and Si1−xGex chemical vapor deposition on silicon surfaces.

Effects of Anodization Current Density on Photoluminescence Properties of Porous Silicon

Photoluminescence (PL) mechanisms of porous Si (PS) have been investigated by comparing Fourier transform infrared (FTIR) absorption structures with PL properties obtained using a He–Cd laser as a photoexcitation source. PS samples investigated were prepared by anodization at different anodization current densities (i a) or by different total electric charge densities (q T). The densities of Si–H and Si–H2 species and the amount of oxygen are constant independent of both i a and q T, and the oxygen is inferred to exist on the topmost surface of the PS layer. On the other hand, the PL intensity depends only on i a and increases drastically with increasing i a. Therefore, there is no relationship between the PL intensity and the hydride densities or the amount of oxygen, indicating that newly formed surface complexes, related to hydrogen or oxygen, including hydrogenated amorphous Si, are not essential to the PL origin, and that the Si atomic configuration, at least within the ∼ 100 Å detection depth in the PL experiment, varies with the change of i a. Macroscopic structures such as porosity and specific surface area were evaluated to be unchanged among the samples. The FTIR and PL results together with this evaluation suggest that photoluminescent elements, which do not affect the macroscopic structures fundamentally, exist in the region of the internal surface of PS pores and the number of elements increases with increasing i a. A quantum-sized crystallite is a most promising photoluminescent element for which these results are well explained. The PL peak energies were almost constant among the samples, which suggests that the minimum size of the quantum-sized crystallites, during anodization, is limited.

Atomic H abstraction of surface H on Si: An Eley–Rideal mechanism?

The abstraction kinetics for atomic hydrogen (Hat) removal of chemisorbed D and atomic deuterium (Dat) removal of chemisorbed H are studied on single crystal Si surfaces. The surface H and D coverages are measured in real time by mass analyzing the recoiled H+ and D+ ion signals. On both Si(100) and Si(111) surfaces, the abstraction reactions are efficient, and have very low activation energies ≂0.5–1 kcal/mol. For abstraction from surfaces containing only monohydride species, the abstraction reaction probability is ≂0.36 times the adsorption rate of Hat or Dat. For the same Hat and Dat exposures, the reaction rates for Hat removal of adsorbed D and Dat removal of adsorbed H are nearly identical. All observations are consistent with a generalized Eley–Rideal abstraction mechanism, and a two-dimensional quantum-mechanical model is used to calculate reaction probabilities for these reactions. According to the model, the activation energies are due to enhanced abstraction rates from excited vibrational states of the adsorbed Si–H or Si–D bond. With SiH2 and SiH3 species present on the surface, the removal rate of H using Dat is decelerated, suggesting that the higher hydrides have a lower cross section for abstraction.

Interpretation of the silicon-hydrogen stretching doublet in a-Si:H hydrogenated amorphous silicon

We studied the correlation between the infrared integrated absorption strengths of the wagging, bending and stretching modes for four hydrogenated amorphous silicon (a-Si:H) series, deposited by rf glow discharge. The strength of these modes was varied by changing the deposition temperature while keeping other parameters fixed. For the first time it is shown that a straightforward correlation exists between these three modes. It is argued that the 2100-cm−1 mode is due to SiH2 bonds and that there is no contribution of SiH species on the inner surface of voids. The vibrational spectrum of a-Si:H can be well described by considering the vibrating dipole as a harmonic oscillator with an effective charge e*=0.44 electrons.

Interpretation of the luminescence quenching in chemically etched porous silicon by the desorption of SiH3 species

The effect of thermal annealing on chemically etched porous silicon was studied by combined photoluminescence, infrared spectrometry, and thermal-desorption spectrometry experiments. The results show that the release of SiH3 and SiF3 entities is a first step in the photoluminescence degradation in porous silicon. These entities desorb before molecular hydrogen.

Ultrahigh vacuum surface analysis of silicon (100) treated in aqueous hydrofluoric acid and buffered hydrofluoric acid solutions

X-ray photoelectron spectroscopy, static secondary ion mass spectroscopy, and thermal desorption spectroscopy were used to study the surface chemistry of Si(100) after wet chemical treatments in dilute HF (1 vol%; pH 3) and buffered HF (15 vol% NH 4OH, 22 vol% HF, 63 vol% H 2O; pH 4). This study was performed in a combined ultrahigh vacuum-electrochemical chamber (UHV-EC) in which sample transfer occured under ultrapure conditions. After wet chemical treatments the Si(100) surface had both SiF and SiH species present, and upon rinsing, SiF was hydrolyzed to SiOH resulting in a lower concentration of SiF. Treatment in buffered HF produces SiF 4 as an etching product. Evidence for this comes from detection of adsorbed SiF - 5 following treatment and transfer to UHV. The ultraclean transfer method used in this study greatly reduces the amount of adsorbed hydrocarbons compared to previous studies involving sample transfer through laboratory air, and thereby allows a more thorough UHV analysis of the surface chemistry of wet etching.

Infrared spectroscopic study of initial stages of ultraviolet ozone oxidation of Si(100) and (111) surfaces

We have investigated the initial stages of UV ozone oxidation of hydrogen-terminated Si(100) and (111) surfaces using infrared spectroscopy in the multiple internal reflection geometry. Spectral features due to intermediate oxidation species such as SiH2(O2) and SiH(O3), which are generated by the attack of the back bonds of a surface Si atom by oxygen, are clearly observed. Upon UV ozone oxidation the concentration of the intermediate oxidation species initially increases and then drops, while that of surface hydride species Si–Hx decreases with increasing the UV ozone exposure, which indicates that at initial stages of oxidation oxygen attacks both of the Si–H and back bonds of a Si atom in the outermost layer. We find that the generation of the intermediate oxidation species exhibits a strong crystallographic orientation dependence; both SiH2(O2) and SiH(O3) are generated on the Si(100) surface, while the generation of the SiH2(O2) species is suppressed on the Si(111) surface.

A comparative study of the initial oxygen and water reactions on germanium and silicon using sims

The initial reactions of oxygen and water with amorphised Ge(100) and Si(100) surfaces at room temperature were investigated using the SIMS (secondary ion mass spectrometry) technique. The intensities of both positive and negative secondary ion species were recorded as a function of oxygen or water pressure (in the range 10 −9 to 5 × 10 −6 mbar) under dynamic dosing conditions. The high power law relationships and secondary ion yields show that Si is more reactive than Ge to initial corrosion by oxygen and water in the pressure range studied. For the oxygen reaction, this increased reactivity is explained by the observation that oxygen adsorbs in a molecular precursor state, and the activation energy for subsequent dissociation is higher for Ge than for Si. Water dissociates to H and OH species on both the Ge and Si surfaces. The water reaction on Si is less rapid than the oxygen reaction since the Si-H species formed inhibit the formation of the SiO 2 corrosion phase due to a site-blocking effect. The Ge-H species, however, appears to be less stable and does not inhibit the oxidation of Ge. Since Ge is significantly less reactive than Si, the GeO 2 corrosion phase does not start to form in the pressure range studied (<5 × 10 −6 mbar) for both oxygen and water pressure.

Silicon-hydrogen (Si-H) bond activation on copper: reaction of silane on Cu(111)

The activation and decomposition of silane on Cu(111) have been studied using Fourier transform infrared (FTIR), Auger electron (AES), and temperature-programmed reaction (TPRS) spectroscopies, as well as low-energy electron diffraction (LEED). Silane dissociatively chemisorbs on Cu(111) at 90 K. Cleavage of the Si-H bond yields two structurally distinct adsorbed silyl fragments. Infrared spectroscopy identifies the predominant intermediates formed under these conditions as being adsorbed SiH[sub 2] and SiH species. The relative and absolute concentrations of these intermediates depend sensitively on the surface coverage of both Si and H, which themselves depend upon the silane exposure. SiH[sub 2] is stable over a wide range of coverage up to 180 K, where it then undergoes Si-H bond cleavage to form surface bound SiH. At higher temperatures, bond scission in the Si-H moiety results in the formation of adsorbed silicon atoms and the desorption of dihydrogen in a peak centered at [approximately]330 K. Auger electron spectra show that the amount of silicon deposited on the Cu(111) surface in this way is approximately one-third of the amount deposited on a stable Cu[sub 3]Si surface. This latter surface is readily formed by carrying out the silane exposure at temperatures above 300 K. 39 refs., 15more » figs., 1 tab.« less